Sedimentary geochemistry and provenance of the Lower and Middle Devonian Laojunshan Formation, the North Qilian Orogenic Belt

Abstract

The Laojunshan Formation is a suite of molasse formed during the rapid uplift of the North Qilian Orogenic Belt (NQOB). Forty-one samples of sandstone have been collected from the Sunan and Minle sections in the western sector and the Gulang and Jingyuan sections in the eastern sector of the NQOB belt. Geochemical analyses of those samples indicated: 1) The MgO+Fe2O3 T and Al2O3/SiO2 values are higher, and K2O/Na2O ratios are lower in the western sector than those in the eastern sector. 2) All of them are depleted in Nb and Ta elements. The samples from the western sector are depleted in Rb element and enriched with Sc, Co, Ni, V, and Cr elements in the Upper Crust-normalized patterns. However, those from the eastern sector are depleted in Sr without enrichments of Sc, Co, Ni, V, and Cr. 3) All of the samples display a right-inclined REE pattern after Chondrite-normalized REE pattern. But LaN/YbN and Eu/Eu* ratios of the samples from the western sector are lower than those of the samples from the eastern sector. These geochemical characteristics suggest the prominent input of mafic clast with minor granitic rocks into the Sunan area, felsic clast into the Gulang and Jingyuan area, and both mafic and felsic clast into the Minle area. The angular shapes of gravels imply that these ill-sorted sediments were deposited near their sources without recycling. Geochemical features above also demonstrated that no major chemical weathering occurred for the western provenance, but deposits in the eastern sector resulted from low or middle degree chemical weathering. Evidences combining tectonic discriminations and comparisons with potential provenances revealed that sediments in the Sunan area were derived mainly from the North Qilian Continental arc, whereas sediments in the Minle, Gulang, and Jingyuan areas were derived not only from the North Qilian Continental arc but also from the basement of the Middle Qilian block. Integrated with the characteristics of development of Silurian and Devonian, these imply that the orogeny of NQOB is diachronous in the trending direction due to the oblique collision.